| Noun | 1. | sacrum - wedge-shaped bone consisting of five fused vertebrae forming the posterior part of the pelvis; its base connects with the lowest lumbar vertebra and its tip with the coccyxbone, os - rigid connective tissue that makes up the skeleton of vertebrates sacral vertebra - one of 5 vertebrae in the human spine that fuse in the adult to form the sacrum pelvic arch, pelvic girdle, pelvis, hip - the structure of the vertebrate skeleton supporting the lower limbs in humans and the hind limbs or corresponding parts in other vertebrates | |